The provided compiler '/usr/bin/gcc' is LLVM based, it is not yet fully supported by ruby and gems, please read `rvm requirements`.

rvm requirements

Right now Ruby requires gcc to compile, but Xcode 4.2 and later no longer ship with gcc. Instead they ship with llvm-gcc (to which gcc is a symlink) and clang, neither of which are supported for building Ruby. Xcode 4.1 was the last version to ship gcc, which was /usr/bin/gcc-4.2.

Xcode 4.1 and earlier:
- Ruby will build fine.

Xcode 4.2 and later (including Command Line Tools for Xcode):
- If you have gcc-4.2 (and friends) from an earlier Xcode version, Ruby will build fine.
- If you don't have gcc-4.2, you have two options to get it:
 * Install apple-gcc42 from Homebrew
 * Install osx-gcc-installer

Homebrew:
If you are using Homebrew, you can install the apple-gcc42 and required libraries from homebrew/dupes:
 brew update
 brew tap homebrew/dupes
 brew install autoconf automake apple-gcc42
 rvm pkg install openssl

This can live side by side with an existing Xcode 4.2+ install or Command Line Tools for Xcode.

brew update
brew tap homebrew/dupes
brew install autoconf automake apple-gcc42

Warning: Could not link apple-gcc42. Unlinking...
Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
You can try again using `brew link apple-gcc42'
==> Summary
/usr/local/Cellar/apple-gcc42/4.2.1-5666.3: 104 files, 75M, built in 23 seconds
kanitw:shell.venturelab.com kanitw$ brew link apple-gcc42
Linking /usr/local/Cellar/apple-gcc42/4.2.1-5666.3... Warning: Could not link apple-gcc42. Unlinking...
Error: Could not symlink file: /usr/local/Cellar/apple-gcc42/4.2.1-5666.3/include/gcc
/usr/local/include is not writable. You should change its permissions.